Reduction of distortion in vacuum tube circuits



Patented Aug. 14, 1.934

STATES REDUCTION OF DISTORTION IN VACUUM TUBE CIRCUITS John G. Kreer,Jr., Bloomfield,- N. J., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationFebruary 2.7, 1932, Serial No. 595,502 r 9 Claims.

, plifiers, modulators and the like, and particularly to the control ofmodulation in such circuits.

It is an object of this invention to suppress effectively modulationproducts and particularly third order modulation products, in signalingsystems employing electric space discharge devices.

'1 It is well known that the electric space discharge tubes employed insignaling systems because of inherent distortion characteristics,produce in their outputs certain modulation products in addition to thewaves of'frequencies which are impressed on their inputs. Thus, when awave of a single frequency is impressed on the input circuit of a spacedischarge tube, the output circuit will contain a Wave of thefundamental frequency and other waves of frequencies harmonical yrelated to the fundamental. If a complex Wave, that is, one composed ofa plurality of frequencies, is impressed on the input of the device, theoutput circuit will contain certain waves of frequencies harmonicallyrelated to the frequensignaling system for producing certaintransformations in waves transmitted thereover, for example,amplification, it is desirable from the standpoint of quality that theeffects of all modulation products produced by the device, be minimized.As the effects of the modulation products in amplification systems onquality of transmission of orders higher than the third are usuallynegligible compared to the effects of the second and third ordermodulation products, if the sec- 0nd and third order products presentare eliminated or reduced to a negligible value by suitable means, thesystem for practical purposes is made distortionless. With some othertubes or other systems such as modulators some higher order product suchas the fourth may predominate.

.Heretofore, it has been found that the amplivalue from the standpointof other considerations, such as maximum power output, minimumreflection losses, etc.

In accordance with the present invention, it has been found that theamplitude of third or higher order modulation products produced byelectric space dischargedevices ofv any type in a system may bereducedto zero or to an unobjectionable amount by means not dependent ona critical value of load impedance. This is accomplished 5; by insertingin the external circuits of the space discharge tubes (not the loadcircuit) means to provide such modulation voltages, of low-er order, assubstantially to suppress unwanted higher order modulation products.

In one embodiment of the invention applicable to a balanced vacuum tubeamplifier circuit, third order modulation products are made to vanish orare reduced to an unobjectionable amount by the insertion of aresistance of appropriate value in the common portion of the platecircuits of the two tubes.

The invention will be better understood from the followingdetaileddescription thereof when read in connection with theaccompanying drawing in which: V

Fig. 1 shows diagrammatically a push-pull vacuum tube amplifier circuitin which the suppression of the third order modulation products isobtained by an impedance of suitable value inserted in the commonportion of the plate circuit of the two tubes; 7 r

Fig. 2 shows a series of curves used to illustrate the invention; 7

Fig. 3 shows diagrammatically a modification of the invention applicableto a push-pull vacuum tube amplifier circuit; and

Fig. 4 showsdiagrammatically an embodiment of the invention applicableto a single tube .amplifier circuit.

If the outputcurrent of a vacuum tube is expressed as a power series ofthe variable component of the grid voltage, it canbeshown that the thirdorder coefficient is a function of the derivatives of thestatic-characteristic at the operating point, the impedance to thefundamental current, the impedance to the second order currents and theimpedance to the third order currents. This may be expressedmathematically as follows:'

the fundamental component being J1=C1e, the second order components are,zzzcze andv the third order components are J3=C'3e where e is theapplied input voltage to the tube; bo1,.b10.,.F102-;. bu, 1220 etc. arecoefficients in the power series expansion of the tube characteristic.

It can be shown mathematically that ii the tube. characteristicsatisfies certain conditions as to continuity, then i 6J1: as." that is,the partial derivative of the plate current with respect to grid.voltage evaluated at the operatingipoint;

a. 12mg bJE Z! that is, the partial derivative of the plate current withrespectto the plate voltage evaluated at the operating point; and thehigher order I) coefiicients are higher order partial derivatives, e.g., bmn is'the (m+n) th partial derivative of the plate current withrespect to plate voltage m times and with respect to grid voltage ntimes.

R1, R2, R3. are the impedances of, the external.

vacuum tube circuit to fundamental, second. or.- der, and third ordermodulation components, respectively. 1

Examination of expression. (1) shows that the third. order coefiicientismade up of two portions, one, indicated by the first term, which. isindependent of second. order currents, and the other, indicated by thesecond term, which is dependent onthe' flow of second order currents. Itis obvious that if the two terms of expression 1) have the same sign andare of the same magnitude, the third order coefiicient, C3, will vanishand there will. be no third order modulation products.

Now, since the denominators of the two terms involved in C3 areidentical, only the numerators need be considered. Then,

New 5 can be put in the form:

6E; R0+R1 6E1, R0 OE obtained directly from. the above definition of thebmns'where R0 equals the internal impedance and a the amplificationfactor of the vacuum tube. It has been found from inspection ofavailable experimental data on a number of commonly used types of threeelement vacuum tubes that for most tubes,

by. On D and 0,

are positive with the latter much larger; also that be n 0E1, OE,

arenegative. Under these conditions [3 will usually be positive. No suchgeneral conclusions can be drawn concerning a, but such values as havebeen computed are positive also. Therefore, if R2 is adjusted until onis equal in magnitude to p, then C3 may be made to vanish.

It so happens that if the impedance to the secand harmonic R2, can bevaried independently of the impedance to the fundamental, R1, the thirdorder coemcient for most types of tubes can be made zero While thefundamental impedance'remains at some value desired. for other circuitreasons. The balanced vacuum tube amplifier illustrateddiagrammaticallyin Fig, 1 presents just such a meansv for varying the impedance totheeven order currents independently of. the impedance to the odd ordercurrents.

In the circuit of Fig. 1., the modulated waves to be amplified areimpressed upon the input circuits of the space discharge devices 1 and 2by means of input transformer 3.. The, devices 1 and 2 are connected inopposing. or push-pull relationship in the. usual manner well known inthe art. The currents in the output circuits of these devices areimpressed by means of output trans,- former 4 upon a suitable load orline which-is represented by the resistance 5..

Assuming a balanced amplifier with an ideal output transformer andidentical tubes, as is. well known the second order currents of thetubes balance out in the output transformer and it acts as a lowimpedance. However, thesecond order currents are in. phase in the commonportion 6 of the plate-filament circuits of the. tubes 1v and 2 and thesecond order voltages of the two tubes. are thus effectivelyconnected-in. parallel sov that the impedance to each is twice thecommon. branch impedance. If the impedance of the commonbranch 6 is R,as indicated in Fig. 1, then The odd order voltages operate in theoutput circuits of the tubes 1 and 2 in series aiding, so if the loadimpedance as reflected through the output transformer 4. is R, then theimpedance to the fundamental currents will be and be quite complex. andwill not, therefore, be given 1 here.

expected that the third order modulation products could be reducedsubstantially to zero merely by obtaining the proper value for theimpedance R.

As shown mathematically above, it would be I of the common branch of theoutput circuits of the two push-pull tubes 1 and 2 by making theresistance element '7 in that branch of suitable value, and experimentswhich have been carried out under direction of the applicant indicatethat with standard types of three element tubes commonly used in suchamplifier circuits, the third order modulation products may be madenegligible by this method.

Fig. 2 shows curves illustrating the variation of third harmonic outputswith different values of resistance used in the mid-branch of a balancedamplifier using Western Electric Company 104-D vacuum tubes. These tubeswere operated with a plate potential of 129 volts and a 22.5 volt gridbias, and a specially designed low. leakage output transformer was used.in Fig. 2 each curve represents the result obtained with a differentinput voltage. The experiments indicated that the best results from thestandpoint of third harmonic reduction in thecase of balanced amplifiersusing 104-D tubes were obtained at most operating points if the tubeswere made to work into an output impedance equal to their internal plateimpedance, although the optimum midbranch resistance varied somewhatwhen the operating point was changed. The operating points tried outvaried from a plate potential of to 130 volts and a grid bias of 2G to40 volts with the lot-D tube. When ordinary output transformers wereused in place of the special low leakage output transformers, it wasfound necessary to insert a reactive network of suitable value in themid-branch to annulthe leakage reactance to insure suitable reduction ofthe third order modulation products for impressed waves over a widefrequency band.

Experiments were also made with balanced amplifiers using WesternElectric Company 101-D vacuum tubes. In the case of these tubes, it wasfound necessary to increase the output impedance to twice the internalplate impedance before a maximum reduction of the third harmonic couldbe obtained with a resistance in the-common branch. In the case of the101-1) tube, it was found that the external resistance in the mid-branchgiving maximum suppression of third order modulation products, was muchlower than that required in the case where Western Electric Company104-13 tubes were used. As in the case of the 104-D tubes it was foundthat the optimum mid-branch resistance varied for different sets oftubes and different operating points.

The experimental results indicated that by properselection of theoperating point, both in the case of the balanced amplifiers employingWestern Electric Company 101-D or 104-D vacuum tubes, on the average aten to twenty decibels reduction of the third harmonic output couldeasily be obtained by properly adjusting the resistance in the commonportion of the plate circuits of the balanced tubes. The exactresistance required is not critical within 10 per cent for anyparticular pair of tubes, but does vary with the operating point of thetubes between zero and one thousand ohms depending also upon the outputat which the maximum suppression is desired. Placing greater suppressionat higher inputs usually will not give as great suppression at the lowerinputs.

In some cases it may be desirable to reduce the third order productsgenerated in the grid circuits of balanced amplifiers when grid current"is permitted to flow, that is, when the amplifier is overloaded. Forthat case, it would'be necessary in order to obtain maximum reduction ofthe third order modulation products, to insert a resistance of propervalue in both the common portion of the input circuit and the commonportion of they output circuit of the balanced tubes.

Furthermore, it is not necessary inxeither the grid or plate circuit toobtain the necessary second order voltages by means of IR dropsas shownin Fig. 1. They may be obtained in any way desirable in the particularcircuit-under considera-' tion as long as the proper phase and amplituderelations are maintained. For example, as illustrated in Fig. 3, asecond order modulator 8 with itsinput-paralleled with the amplifierinput and its output in series with the mid-branch ,of' the amplifieroutput might be used 'to provide. the proper second order voltages inthe plate circuits of the tubes.

Fig. 4. shows a modification-of the invention embodied in an amplifiercomprising a single space discharge tube. The waves to be amplified areimpressed upon the input circuit of the space discharge tube 9 by meansof input transformer 10 and the amplified waves in the output circuit ofthe tube 9 are impressed by means of output transformer 11 upon any typeof load circuit or line represented in the figure by the resistance 12.Space current for the tube 9 is supplied by plate battery 13 and asuitable biasing potential is supplied to the grid of the tube 9 bymeans of battery 14. A suitable selective network 15 such as a resonantcircuit tuned to reject the second order products while transmitting thefundamental components of the waves impressed on the input of the tube9, is connected in the output circuit of the tube 9 between the plateand filament thereof. Connected directly across the plate and filamentof the tube 9 is a circuit 16 comprising selective network 17, such as aresonant circuit, and a variable resistance 18 inseries. The selectivenetwork 17 should be of such a nature as to have a free transmissionrange including the second order modulation products generated in tube9, and also as to present a high impedance to other modulation productsand'the fundamental.

By suitable adjustment of the resistance 16, the

impedance of the circuit 14 to the second har-- monic or second ordermodulation products may be made of the critical value at which the thirdharmonic or the'third order modulation products generated by the tube '7are suppressed or reduced to low amplitudes, so that substantially,only'the fundamental waves'will be impressed by the output transformer 9upon the load circuit 10.

Although this invention has been illustrated and described as applied toa particular system, it is not to be limited thereto, but only inaccordance with the scope of the following claims.

What is claimed is:

1. In combination in a wave transmission system, an electric spacedischarge device having.

second .order modulation products of waves from tem,a pair of spacedischarge devices having input and output circuits and connected inpushpull relation" with respectto an incoming and outgoing wavetransmission circuit, and means common to the output circuits of saiddevices and independent of the impedance of the outgoing circuit,reducing in said outgoing circuit third or higher order modulation ofthe waves receivedover said incoming circuit.

a 3. In combination-in a wave transmission system; a pair of spacedischarge devices connected between an .incoming and an outgoingcircuit, and having a portion of their input and their output circuitsin common, and means for reducing insaid outgoing circuit the amplitudesof third order modulation products of waves impressed on :the inputcircuits of saidspace discharge de vices from said incoming circuit,said means comprising means proportioning the impedance of the commonportion of the output circuits of said devices to second ordermodulation products oftheimpressed waves to the critical value at which.the amount of third order modulationcurrenttransmitted to'said outgoingcircuit is a minimum. 4. In a wave transmission system, an incomingcircuit and an outgoing circuit, a pair of space discharge deviceshaving their input and output circuits connected in push-pull relationwith saidincoming circuit and said outgoing circuit, respectively, saiddevices being balanced so as to suppress effectively insaid outgoingcircuit second order modulationproducts of the waves from the incomingcircuit and means common to the output circuits ofsaid devices andindependent of the impedance of said outgoing circuit substantiallypreventing transmission to said outgoing circuit of third ordermodulation products of the waves from said incoming circuit.

5. The system of claim 4 and in which said means substantiallypreventing transmission of third order modulation products to saidoutgoing circuit comprises means making the impedance of the commonportion of the output circuits of said devices to second ordermodulation products'of the critical value at which third ordermodulation currents are substantially suppressed from said outgoingcircuit. 7 V

6. An amplifier comprising a pair of space discharge devices havingportions of their input and output circuits in common 'so that saiddevices areconnected in push-pull relation with respect to an incomingand outgoing wave transmission circuit, said devices being balanced soas .to sup-- press effectively from the outgoing circuit thethe-incoming circuit, and resistance means in the common portion of theoutput circuits of said devices proportioning the impedance of said com-.mon portion to the second order modulation products therein to thecritical value at which the third order modulation products of saidwavesin the respective output circuits of said devices substantiallyneutralizeeach other.

'7. In combination ina wave transmission system, a pair ofthree-electrode space discharge devices, said devices having a portionof their cathode-grid circuits and a portion of their anode-cathodecircuits in common, so as to be connected in push-pull relation withrespect to a source of waves to be repeated and an outgoing circuit,said devices having substantially similar characteristics with respectto second order modulation products appearing in the in-'- dividualportions of their anode-cathode circuits whereby transmission of said.second'order products to said outgoing circuit is substantiallyprevented, and a resistance element in the common portion of the outputcircuits of said devices, and of such value as to make the impedance ofsaid common portion to second order modulation products appearingtherein of the critical Value which will reduce the. amplitudes of thethird order modulation products transmitted to said outgoing circuit toa minimum.

8. In combination in a wave transmission system, a three-electrodeelectric space discharge device having a space path, a cathode-controlelectrode circuit and a cathode-anode circuit, means for impressingwaves to be repeated on said cathode-control electrode circuit, a'loadcircuit coupled to said cathode-anode circuit so as to receive therepeated waves, a circuit connected effectively in shunt with the spacepath of said device and comprising a resonant circuit tuned to secondorder modulation products of said waves, and means for reducing theeffects of third order modulation products of said waves in said loadcircuit, said means comprising means for proportioning the impedance ofsaid shunt circuit with respect to the second order modulation productsof said waves to the critical value at which the amplitudes of the thirdorder modulation products transmitted to said load circuit are aminimum.

9. In combination in a wave transmission system, a, three-electrodeelectric space discharge device comprising a space path, input andoutput circuits, means for impressing on said input circuit of saiddevice waves to be repeated, a loadcircuit, means for impressing therepeated tion products of said waves in said load circuit,

and means for reducing the effects of modula-- tion productsof saidwaves in said load ciruit,

comprising a selective network in said output cir-' cuit ofsuch natureas to suppress second order modulation components of said waves, acircuit in parallel with said output circuit and directly in shunt withsaid space path, a selective net-' Work in said parallel circuit of sucha nature as to pass the second order modulation components of saidwaves, and means for proper CERTIFICATE OF CORRECTION.

Patent No. 1,970,325. August 14, 1934.

JOHN G. KREER, IRE

it is hereby eertiiicd that error appears in the printed specificationof the above numbered patent requiring correction as ioiiows: Page 4,claim 9, strike out line 122 and insert instead the Words and commawaves in said output circuit on said load circuit,; and that the saidLetters Patent should he read with this correction therein that the samemay conform to the record of the case in the Patent Office.

Signed and sealed this 17th day of December, A. i). 1935.

Leslie Frazer (Seal) Acting Commissioner oi: Pateots,

{M W M

